I've no idea what happened to the other two eggs. There is no evidence of them on the ground below and for sure they didn't hatch and fly off. Its been really cold here. at least the remaining egg appears to be growing. Is that even possible?

No I just checked. There is still that last one safe and sound. The broken egg was found about 50 ft away. So maybe it was stolen and then dropped by another bird like some have suggested.
It is finally supposed to warm up here by Thursday. So maybe that will be the big day.

Except for that weird heat wave in March, it has felt like a steady 3-5 degrees since November. No winter, but no spring either. I`m so sick of being cold all the time.